The present invention relates to a hydraulic pressure modulation device for a power shift clutch mechanism employed in industrial machines such as a forklift, a shovel loader, or the like.
Generally, a power shift clutch mechanism has several hydraulic clutches such as a forward driving clutch, a reverse driving clutch, a first (low speed) clutch and a second clutch. These clutches are connected to a hydraulic pressure source through a selector valve connected to a shift lever, and are operable to be engaged and released by operating the shift lever. However, a known mechanism has such a disadvantage that smooth speed changing and starting may not be possible, because a hydraulic pressure introduced to the clutch increases very rapidly at substantially the same time as shifting occurs, which causes sudden engaging of the clutch.
In order to avoid the sudden increasing of the pressure, some of the known mechanisms have employed oil pressure modulation devices disposed in hydraulic lines. A known modulation device comprises, for example, combined large and small pistons, or an accumulator and a choke combined together.
However, such known modulation devices can not perform the intended modulation operation when the shifting operation is rapidly repeated.
Accordingly, it is an object of the invention to provide an improved hydraulic pressure modulation device overcoming the above-noted disadvantages.
Other and further objects, features and advantages of the invention will appear more fully from the following description of the preferred embodiments of the invention.